Well then, you sure don't want a taller tire (60 series). If you are keeping the same gears, you will need to lower your tire diameter. Tire sizes along the lines of what Stealthwedge67 recommended. If the widest you could fit is a 275 tire width, see if there is a 50 or 45 series profile, which would give you a lower sidewall/overall diameter. This would give you both, lower 60 ft. and higher trap rpm.